Set the oven temperature to 180°C. Grease a 20 cm round baking tin with oil and line the base with baking paper. Lightly dust the sides of the pan with khapli wheat flour.

In a bowl, whisk together khapli wheat flour, cinnamon powder, baking powder, baking soda and salt. Mix well so the raising agents and cinnamon are evenly distributed.

In another bowl, whisk the eggs and brown sugar until smooth. Add oil, milk and vanilla extract, then whisk until the mixture is evenly combined.

Gradually add the dry ingredients to the wet mixture, folding gently until just combined. Avoid overmixing, as this can make the cake dense. The batter should be thick but pourable.

Use a spatula to fold the diced apples and walnuts into the mixture. Mix just enough so that the apples and walnuts are evenly distributed and not too mashed up.

Pour the batter into the cake pan and level it out. Use the apple slices set aside earlier to cover the batter. Mix the brown sugar with cinnamon and sprinkle it evenly on top.

Put the cake in a preheated oven at 180°C for 35-40 minutes, or until a skewer comes out clean or with only a few crumbs attached. The top of the cake should be golden and should be pulling away from the sides of the cake pan.

Once done, remove the cake from the oven and let it cool for 10 minutes before transferring it to a cooling rack. This is an important final step that sets the crumb of the cake while retaining its moisture.